This application claims the benefit of Japanese Application No. 2002-046828 filed Feb. 22, 2002.
The present invention relates to a static magnetic field inhomogeneity distribution measuring method, static magnetic field homogenizing method, MR (magnetic resonance) data collecting method, and MRI (magnetic resonance imaging) apparatus, and more particularly to a static magnetic field inhomogeneity measuring method, static magnetic field homogenizing method, MR data collecting method, and MRI apparatus by which the effect of residual magnetization caused by gradient pulses can be thoroughly reduced.
A known static magnetic field inhomogeneity distribution measuring sequence SQJ is shown in FIG. 7.
The static magnetic field inhomogeneity distribution measuring sequence SQJ applies an RF pulse P1 and a slice selective gradient Ss1; subsequently applies a slice rephasing gradient Sr1xe2x80x2 having an intensity (amplitude) equal to that of the slice selective gradient Ss1; applies a phase encoding gradient Pe1; applies a frequency dephasing gradient Fd1xe2x80x2 having an intensity equal to that of a readout gradient Ro1; subsequently collects first MR data while applying the readout gradient Ro1; then, applies a phase rewinding gradient Pr1xe2x80x2 having an intensity equal to that of the phase encoding gradient Pe1; and further applies a killer gradient Sk1xe2x80x2 having an intensity greater than that of the slice selective gradient. Subsequent to the first gradient echo sequence, a second gradient echo sequence in which the echo time is shifted by xcex4t collects second MR data. Then, the distribution of static magnetic field inhomogeneity is measured based on the phase difference between the first and second MR data.
Moreover, Japanese Patent Application Laid Open No. 2001-54510 discloses means for suppressing variation of residual magnetization in a magnetization conditioning plate, for example, in an MRI apparatus, which variation depends upon the history of gradient pulse application, in which means:
(1) a residual magnetization reducing pulse is applied immediately after the phase encoding gradient;
(2) a residual magnetization reducing pulse is applied immediately after the phase rewinding gradient;
(3) a residual magnetization reducing pulse is applied immediately after the killer gradient;
(4) the intensity (amplitude) of the slice rephasing gradient is made about half of that of the slice selective gradient; and
(5) the intensity of the frequency dephasing gradient is adjusted.
FIG. 8 is a magnetization characteristic graph for explaining the effect of residual magnetization caused by the gradient pulses in the static magnetic field inhomogeneity distribution measuring sequence SQJ shown in FIG. 7. It should be noted that the graph is presented merely for explanation of a concept, and does not limit the present invention.
Considering first only the effect of the gradient pulses applied to the slice axis in general, when the magnetization lies at a point a of a magnetization B0 corresponding to a static magnetic field strength H0, application of the slice selective gradient Ss1 and slice rephasing gradient Sr1xe2x80x2 by the first gradient echo sequence causes the magnetization to move from the point a, through points b, c and e, to a point f. Then, when the killer gradient Sk1xe2x80x2 is applied, the magnetization moves from the point f through the point b, enters a major loop, and travels through a point Bxe2x80x2 to a point cxe2x80x2. In the second gradient echo sequence, the magnetization varies along another minor loop containing the point cxe2x80x2.
Considering next only the effect of the gradient pulses applied to the phase axis in general, when the magnetization lies at the point a of a magnetization B0 corresponding to a static magnetic field strength H0, application of the phase encoding gradient Pe1 by the first gradient echo sequence causes the magnetization to move from the point a through the point b to the point c, and then application of the phase rewinding gradient Pr1xe2x80x2 causes the magnetization to move from the point c through the point e to the point f. Next, when the phase encoding gradient Pe2 is applied by the second gradient echo sequence, the magnetization moves from the point f through the point b to the point c, and when the phase rewinding gradient Pr2xe2x80x2 is then applied, the magnetization moves from the point c through the point e to the point f.
Next, considering only the effect of the gradient pulses applied to the frequency axis, when the magnetization lies at the point a of a magnetization B0 corresponding to a static magnetic field strength H0, application of the frequency dephasing gradient Fd1xe2x80x2 and readout gradient Ro1 by the first gradient echo sequence causes the magnetization to move from the point a, through the points e, f, and b, to the point c. The magnetization varies in a similar manner in the second gradient echo sequence.
As described above, the conventional static magnetic field inhomogeneity distribution measuring sequence SQJ poses a problem in that the distribution of static magnetic field inhomogeneity cannot be accurately measured because magnetization varies due to gradient pulses.
Consequently, correct shimming cannot be achieved, and the image quality may be significantly degraded especially when conducting an imaging method utilizing the resonance frequency difference between water and fat, such as a CHESS (chemical shift selective imaging) method, in an MRI apparatus with a medium-to-low magnetic field (0.3-0.5 T).
To solve the problem, the inventors of the present invention studied use of means disclosed in Japanese Patent Application Laid Open No. 2001-54510, but the distribution of static magnetic field inhomogeneity cannot be highly accurately measured only by this means. Further investigation was therefore made to find new means for suppressing variation of residual magnetization that depends upon the history of gradient pulse application, and completed the present invention.
An object of the present invention is to provide a static magnetic field inhomogeneity measuring method, static magnetic field homogenizing method, MR data collecting method, and MRI apparatus by which the effect of residual magnetization caused by gradient pulses can be thoroughly reduced.
In accordance with its first aspect, the present invention provides a static magnetic field inhomogeneity distribution measuring method characterized in comprising: collecting first MR data from an echo focused by a first gradient echo sequence in which the intensity of a killer gradient applied to a slice axis is made equal to that of a slice selective gradient, the intensity of a slice rephasing gradient is made half or about half of that of the slice selective gradient, the intensity of a phase rewinding gradient is made half or about half of that of a phase encoding gradient, and the intensity of a frequency dephasing gradient is made double or about double of that of a readout gradient; collecting second MR data from an echo focused by a second gradient echo sequence in which the echo time is shifted by xcex4t relative to said first gradient echo sequence; and measuring the distribution of static magnetic field inhomogeneity based on the phase difference between said MR data.
Conventionally, the intensity of the killer gradient applied to the slice axis is greater than that of the slice selective gradient. Therefore, the variation of residual magnetization due to the killer gradient is large. Thus, Japanese Patent Application Laid Open No. 2001-54510 described above proposes application of a residual magnetization reducing pulse immediately after the killer gradient.
In contrast, the intensity of the killer gradient applied to the slice axis is made equal to that of the slice selective gradient in the static magnetic field inhomogeneity distribution measuring method of the first aspect. Therefore, the variation of residual magnetization by the killer gradient is limited within variation of residual magnetization by a slice selective gradient applied after the killer gradient, and such variation can be sufficiently dealt with by means for suppressing variation of residual magnetization by a slice selective gradient, which will be described next. Thus, the need to apply the residual magnetization reducing pulse immediately after the killer gradient is eliminated.
Moreover, in the static magnetic field inhomogeneity distribution measuring method of the first aspect, the intensity of the slice rephasing gradient is made half or about half of that of the slice selective gradient. Thus, the variation of residual magnetization due to the slice selective gradient is suppressed, as disclosed in Japanese Patent Application Laid Open No. 2001-54510.
That is, the effect of residual magnetization by gradient pulses applied to the slice axis is reduced.
Next, for the phase axis, Japanese Patent Application Laid Open No. 2001-54510 described above proposes application of a residual magnetization reducing pulse immediately after the phase encoding gradient.
In contrast, the intensity of the phase rewinding gradient is made half or about half of that of the phase encoding gradient in the static magnetic field inhomogeneity distribution measuring method of the first aspect. Thus, the effect of residual magnetization by gradient pulses applied to the phase axis is reduced, as will be described below with reference to FIG. 3.
Next, for the frequency axis, the intensity of the frequency dephasing gradient is made double or about double of that of the readout gradient. Thus, the effect of residual magnetization by gradient pulses applied to the frequency axis is reduced, as disclosed in Japanese Patent Application Laid Open No. 2001-54510.
Ultimately, since the effects of residual magnetization by gradient pulses applied to all the gradient axes are reduced in the static magnetic field inhomogeneity distribution measuring method of the first aspect, the distribution of static magnetic field inhomogeneity can be highly accurately measured.
In accordance with its second aspect, the present invention provides a static magnetic field inhomogeneity distribution measuring method characterized in comprising: collecting first MR data from an echo focused by a first gradient echo sequence in which the intensity of a slice rephasing gradient is made half or about half of that of a slice selective gradient, the intensity of a slice rewinding gradient is made half or about half of that of a slice encoding gradient, the intensity of a phase rewinding gradient is made half or about half of that of a phase encoding gradient, the intensity of a frequency dephasing gradient is made double or about double of that of a readout gradient, and the intensity of a killer gradient applied to a frequency axis is made equal to that of the readout gradient and the killer gradient is applied subsequent to the readout gradient; collecting second MR data from an echo focused by a second gradient echo sequence in which the echo time is shifted by xcex4t relative to said first gradient echo sequence; and measuring the distribution of static magnetic field inhomogeneity based on the phase difference between said MR data.
In the static magnetic field inhomogeneity distribution measuring method of the second aspect, a slice rewinding gradient is applied which has an intensity half of or about half of that of the slice encoding gradient applied for 3D imaging. Thus, the effect of residual magnetization by the slice encoding and rewinding gradients is reduced, as will be described later with reference to FIG. 3. Moreover, a killer gradient applied to the frequency axis has an intensity equal to that of the readout gradient and the killer gradient is applied subsequent to the readout gradient. Therefore, variation of residual magnetization by the killer gradient can be regarded as being merged with residual magnetization by the readout gradient. Since the intensity of the frequency dephasing gradient is made double or about double of that of the readout gradient, the effect of residual magnetization by gradient pulses applied to the frequency axis is reduced, as disclosed in Japanese Patent Application Laid Open No. 2001-54510. Other features are similar to those of the static magnetic field inhomogeneity distribution measuring method of the first aspect.
Ultimately, since the effects of residual magnetization by gradient pulses applied to all the gradient axes are reduced also in 3D imaging in the static magnetic field inhomogeneity distribution measuring method of the second aspect, the distribution of static magnetic field inhomogeneity can be highly accurately measured.
In accordance with its third aspect, the present invention provides a static magnetic field inhomogeneity distribution measuring method characterized in comprising: collecting first MR data from an echo focused by a first gradient echo sequence in which the intensity of a slice rephasing gradient is made half or about half of that of a slice selective gradient, the intensity of a phase rewinding gradient is made half or about half of that of a phase encoding gradient, the intensity of a frequency dephasing gradient is made double or about double of that of a readout gradient, and the intensity of a killer gradient applied to a frequency axis is made equal to that of the readout gradient and the killer gradient is applied subsequent to the readout gradient; collecting second MR data from an echo focused by a second gradient echo sequence in which the echo time is shifted by xcex4t relative to said first gradient echo sequence; and measuring the distribution of static magnetic field inhomogeneity based on the phase difference between said MR data.
In the static magnetic field inhomogeneity distribution measuring method of the third aspect, a killer gradient applied to the frequency axis has an intensity equal to that of the readout gradient and the killer gradient is applied subsequent to the readout gradient. Therefore, variation of residual magnetization by the killer gradient can be regarded as being merged with residual magnetization by the readout gradient. Since the intensity of the frequency dephasing gradient is made double or about double of that of the readout gradient, the effect of residual magnetization by gradient pulses applied to the frequency axis is reduced, as disclosed in Japanese Patent Application Laid Open No. 2001-54510. Other features are similar to those of the static magnetic field inhomogeneity distribution measuring method of the first aspect.
Ultimately, since the effects of residual magnetization by gradient pulses applied to all the gradient axes are reduced in the static magnetic field inhomogeneity distribution measuring method of the third aspect, the distribution of static magnetic field inhomogeneity can be highly accurately measured.
In accordance with its fourth aspect, the present invention provides a static magnetic field homogenizing method characterized in comprising: collecting MR data for conducting shimming by the static magnetic field inhomogeneity distribution measuring method having the aforementioned configuration.
In the static magnetic field inhomogeneity distribution measuring method of the fourth aspect, since MR data is highly accurately measured, correct shimming can be conducted, and the image quality is improved especially when conducting an imaging method utilizing the resonance frequency difference between water and fat, such as the CHESS method, in an MRI apparatus with a medium-to-low magnetic field.
In accordance with its fifth aspect, the present invention provides an MR data collecting method characterized in that: the intensity of a phase rewinding gradient is made half or about half of that of a phase encoding gradient.
In the MR data collecting method of the fifth aspect, the intensity of the phase rewinding gradient is made half or about half of that of the phase encoding gradient. Thus, the effect of residual magnetization by gradient pulses applied to the phase axis is reduced, as will be described later with reference to FIG. 3.
In accordance with its sixth aspect, the present invention provides an MR data collecting method characterized in that: the intensity of a slice rewinding gradient is made half or about half of that of a slice encoding gradient.
In the MR data collecting method of the sixth aspect, a slice rewinding gradient having an intensity half or about half of that of a slice encoding gradient applied for 3D imaging is applied. Thus, the effect of residual magnetization by the slice encoding gradient and slice rewinding gradient is reduced, as will be described later with reference to FIG. 3.
In accordance with its seventh aspect, the present invention provides an MR data collecting method characterized in comprising: applying a killer gradient having an intensity equal to that of a slice selective gradient to a slice axis.
In the MR collecting method of the seventh aspect, the intensity of the killer gradient applied to the slice axis is made equal to that of the slice selective gradient. Therefore, variation of residual magnetization by the killer gradient is limited within variation of residual magnetization by the slice selective gradient applied after the killer gradient, and no special measure for dealing with the variation of residual magnetization caused by the killer gradient is needed.
In accordance with its eighth aspect, the present invention provides an MR data collecting method characterized in comprising: applying a killer gradient having an intensity equal to that of a readout gradient to a frequency axis subsequent to the readout gradient.
In the MR data collecting method of the eighth aspect, a killer gradient applied to the frequency axis has an intensity equal to that of the readout gradient and the killer gradient is applied subsequent to the readout gradient. Therefore, since variation of residual magnetization by the killer gradient can be regarded as being merged with residual magnetization by the readout gradient, no special measure for dealing with the variation of residual magnetization caused by the killer gradient is needed.
In accordance with its ninth aspect, the present invention provides an MRI apparatus characterized in comprising: RF pulse transmitting means; gradient pulse applying means; MR signal receiving means; first MR data collecting means for, by controlling the aforesaid means, collecting first MR data from an echo focused by a first gradient echo sequence in which the intensity of a killer gradient applied to a slice axis is made equal to that of a slice selective gradient, the intensity of a slice rephasing gradient is made half or about half of that of the slice selective gradient, the intensity of a phase rewinding gradient is made half or about half of that of a phase encoding gradient, and the intensity of a frequency dephasing gradient is made double or about double of that of a readout gradient; and second MR data collecting means for collecting second MR data from an echo focused by a second gradient echo sequence in which the echo time is shifted by xcex4t relative to said first gradient echo sequence.
In the MRI apparatus of the ninth aspect, the static magnetic field inhomogeneity distribution measuring method of the first aspect can be suitably implemented.
In accordance with its tenth aspect, the present invention provides an MRI apparatus characterized in comprising: RF pulse transmitting means; gradient pulse applying means; MR signal receiving means; first MR data collecting means for, by controlling the aforesaid means, collecting first MR data from an echo focused by a first gradient echo sequence in which the intensity of a slice rephasing gradient is made half or about half of that of a slice selective gradient, the intensity of a slice rewinding gradient is made half or about half of that of a slice encoding gradient, the intensity of a phase rewinding gradient is made half or about half of that of a phase encoding gradient, the intensity of a frequency dephasing gradient is made double or about double of that of a readout gradient, and the intensity of a killer gradient applied to a frequency axis is made equal to that of the readout gradient and the killer gradient is applied subsequent to the readout gradient; and second MR data collecting means for collecting second MR data from an echo focused by a second gradient echo sequence in which the echo time is shifted by xcex4t relative to said first gradient echo sequence.
In the MRI apparatus of the tenth aspect, the static magnetic field inhomogeneity distribution measuring method of the second aspect can be suitably implemented.
In accordance with its eleventh aspect, the present invention provides an MRI apparatus characterized in comprising: RF pulse transmitting means; gradient pulse applying means; MR signal receiving means; first MR data collecting means for, by controlling the aforesaid means, collecting first MR data from an echo focused by a first gradient echo sequence in which the intensity of a slice rephasing gradient is made half or about half of that of a slice selective gradient, the intensity of a phase rewinding gradient is made half or about half of that of a phase encoding gradient, the intensity of a frequency dephasing gradient is made double or about double of that of a readout gradient, and the intensity of a killer gradient applied to a frequency axis is made equal to that of the readout gradient and the killer gradient is applied subsequent to the readout gradient; and second MR data collecting means for collecting second MR data from an echo focused by a second gradient echo sequence in which the echo time is shifted by xcex4t relative to said first gradient echo sequence.
In the MRI apparatus of the eleventh aspect, the static magnetic field inhomogeneity distribution measuring method of the third aspect can be suitably implemented.
In accordance with its twelfth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in further comprising: static magnetic field inhomogeneity distribution measuring means for measuring the distribution of static magnetic field inhomogeneity based on the phase difference between said first and second MR data.
In the MRI apparatus of the twelfth aspect, the static magnetic field inhomogeneity distribution measuring method of the firstxe2x80x94third aspects can be suitably implemented.
In accordance with its thirteenth aspect, the present invention provides an MRI apparatus comprising RF pulse transmitting means, gradient pulse applying means, and MR signal receiving means, characterized in that: said gradient pulse applying means makes the intensity of a phase rewinding gradient half or about half of that of a phase encoding gradient.
In the MRI apparatus of the thirteenth aspect, the MR data collecting method of the fifth aspect can be suitably implemented.
In accordance with its fourteenth aspect, the present invention provides an MRI apparatus comprising RF pulse transmitting means, gradient pulse applying means, and MR signal receiving means, characterized in that: said gradient pulse applying means makes the intensity of a slice rewinding gradient half or about half of that of a slice encoding gradient.
In the MRI apparatus of the fourteenth aspect, the MR data collecting method of the sixth aspect can be suitably implemented.
In accordance with its fifteenth aspect, the present invention provides an MRI apparatus comprising RF pulse transmitting means, gradient pulse applying means, and MR signal receiving means, characterized in that: said gradient pulse applying means applies a killer gradient having an intensity equal to that of a slice selective gradient to a slice axis.
In the MRI apparatus of the fifteenth aspect, the MR data collecting method of the seventh aspect can be suitably implemented.
In accordance with its sixteenth aspect, the present invention provides an MRI apparatus comprising RF pulse transmitting means, gradient pulse applying means, and MR signal receiving means, characterized in that: said gradient pulse applying means applies a killer gradient having an intensity equal to that of a readout gradient to a frequency axis subsequent to the readout gradient.
In the MRI apparatus of the sixteenth aspect, the MR data collecting method of the eighth aspect can be suitably implemented.
According to the static magnetic field inhomogeneity distribution measuring method, static magnetic field homogenizing method, MR data collecting method, and MRI apparatus of the present invention, the effect of residual magnetization caused by gradient pulses can be thoroughly reduced. As a result, correct shimming can be conducted, and the image quality is improved especially when conducting an imaging method utilizing the resonance frequency difference between water and fat, such as the CHESS method, in an MRI apparatus with a medium-to-low magnetic field.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.